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ZL2PD Mini Metal Detector

This
very simple metal detector uses a coil wound on a small chunk of
ferrite rod. A cheap piezo speaker allows the changing tone in the
presence of metal to be clearly heard.

Introduction

When
I'm at home, Saturdays are often spent fixing things around the house,
with the occasional bit of redecoration thrown in for good measure. A
typical morning some time back found me facing a wall, my hammer in one
hand and a large, and heavy, picture frame in the other. My mission -
To hang a picture on the wall.

The wall was a good example of what some artists might call a blank
canvas. It was a featureless and dead flat, painted with matt pale
cream paint, and made from plaster board fixed over a standard
wooden frame. Most times, you can look at these walls at certain angles
against the light and spot where the plaster board has been nailed
down. Or you can tap at it with a screwdriver handle or a rubber hammer
handle and hear the sound change as you tap on a hollow section or
right on a wooden stud. If I could find one of those studs, I'd stand a
better chance of nailing the picture hook in a spot which could
reliably support the heavy picture frame.

Well, I stared at that wall for a bit, first one way, then the other. I
tapped at it here and there trying to listen for the sweet spot - the
place to place the picture hanger. But that wall was pretty much
perfect. Heavy plasterboard, well fastened to the frame. Somehow, I had
to find those wooden studs. Maybe I could locate the galvanized nails
fastening that wallboard using a metal detector!

This little mini metal detector was the solution I came up with in less
than an hour.

Design

Three
basic types of metal detectors exist - Beat frequency oscillator (BFO),
field balance, and pulse induction. Other more esoteric types also
exist, including capacitive field and 'radar' detectors, but those
three are the ones most commonly build by enthusiasts.

This circuit is one of the BFO variety. A fixed oscillator is mixed
with a second oscillator whose frequency varies when metal objects
approach this oscillator's search coil. The mixing process results in a
difference tone, or "beat" frequency, being heard. The unit is built
around a single CMOS IC operated from a 9V battery. It draws about 8mA
during use, but since that is limited to the odd 5 minutes three or
four times a year, I've yet to replace the battery.

The
fixed oscillator in this metal detector uses a cheap 455 kHz ceramic
resonator recycled from an old TV remote control. This forms the
feedback path around one of the four gates in the CMOS CD4001 chip
which is configured as an inverter.

Note: Click on the diagram (shown on the
left) with your right hand mouse button to see the circuit diagram at
full scale

The
variable oscillator uses a similar circuit, but the feedback path this
time includes the search coil. It is made from 120 turns of fine 34 SWG
enamelled copper wire wound directly onto a 15mm length of ferrite rod.
The rod is about 10mm in diameter. I used a small piece cut from a
broken broadcast (AM) radio ferrite rod.

To cut off a piece like this, score around the ferrite rod using a
small triangular file. It needs to be more than a scratch - probably
about 0.5mm deep will do the trick. Then, while holding the larger end
of the ferrite rod in a vise, lightly tap down on the end of the rod
with a small hammer. The rod will break (almost always) quite cleanly
at the scored line.

The two oscillators are mixed in the third gate, and the resulting
mixture of output frequencies are passed through a low pass RC filter
(R1 and C9) to the last gate. This gate is configured as a high gain
amplifier, and drives a small piezo speaker. These speakers are about
25mm in diameter, less than 0.5 mm thick. (The picture to the left
shows two of these - One has a white plastic cover, the other is just a
bare piezo element)

I find (recover) most of mine from musical greeting cards, although
quite a few of mine also were used as ringers in old cordless phone
handsets.

The audio level from this circuit and the piezo speaker is more than
adequately loud. You won't want to leave it on any longer than
necessary!

Construction

I built
my version of this on a scrap of PCB measuring 20mm wide by 50mm long,
in "ugly bug" style with the chip soldered "pins-up" onto the PCB. The
other parts were soldered into place using the chip and the PCB to hold
things together. (This really was a "quick and cheap" project!) No
parts are at all critical - The values shown are just those which I had
right on hand at the time. All capacitors are disc ceramic types,
although C9 and C10 can be mylar ('greencap') types.

The battery was temporarily mounted onto the back of the PCB with a
nylon ziptie. I used a pushbutton for the on/off switch. It must be
continually pressed to keep power on the circuit. That way, I can't
accidentally leave it turned on.

The search coil was left to dangle by its leads from one end of the PCB
scrap. I've never bothered to build it into any better or tidier
fashion. I just don't use it often enough to be bothered, but for those
making greater use of the device, it can be built into a small plastic
box.

Operation

Once
built, the variable frequency oscillator is adjusted using the 60pF
trimmer capacitor. This is very sensitive, and a wide range of "tones"
(I use this word with some care since it describes a set of audible
sounds which musicians would not usually classify as tones) can be
produced as this trimmer is adjusted. I found it easiest to adjust
using a plastic screwdriver. Just adjust it for a reasonably loud
audible tone. Moving a metal object (such as a small nail or the end of
a metal screwdriver) near to the search coil will cause this tone to
change wildly.

In service, the search coil is moved across the painted wall, about
10mm from the surface. The tone will abruptly change as the coil passes
close to a hidden nail. The small diameter coil allows the location of
the nail to be found with satisfactory precision.